HPT - faint lines???

I am really curious as to why a test would show a faint +ve just for AF to show up the next day????

This happened with TTC#1 and has happenend again a month ago whilst we are TTC#2.....

Have just had spotting on day 23 (never been shorter that 28 - past 2 months have been 32 day cycle) so am not too keen to test because even if I do get a faint +ve (if say it's implantation bleeding...not holding breath ), I can't really trust that can I